Mobile IP:

• Mobile IP allows users to move from one network to another without loosing connectivity. It allows a host to move to a foreign network, but still maintains its home IP address.
• Mobile IP is an Internet Engineering Task Force (IETF) standard communications protocol that is designed to allow mobile device users to move from one network to another while maintaining a permanent IP address.
• Mobile devices have IP addresses that are associated with one network and moving to another network means changing IP address.
• Using the mobile IP system it allows users to achieve it and at the same time make the underlying process transparent for a user.
• Mobile IP is scalable for the Internet because it is based on IP and any media that can support IP can support Mobile IP.
• Mobile IP uses two IP addresses: a fixed home address and a care-of address that changes at each new point of attachment.
• Mobile IP has the following three components: Mobile Node, Home Agent and Foreign Agent.
• The Mobile IP process has three main phases to work which are Agent Discovery, Registration and Tunneling.

Agent Discovery :

• A Mobile Node discovers its Foreign and Home Agents during agent discovery. Agent Discovery is the method by which a mobile node determines whether it is currently connected to its home network or to a foreign network, and by which a mobile node can detect when it has moved from one network to another.
• When connected to a foreign network, the methods also allow the mobile node to determine the foreign agent care-of address being offered by each foreign agent on that network.
• Mobile IP extends ICMP Router Discovery as its primary mechanism for Agent Discovery. An Agent Advertisement is formed by including a Mobility Agent Advertisement Extension in an ICMP Router advertisement message

Registration:

• The Mobile Node registers its current location with the Foreign Agent and Home Agent during registration. Mobile IP registration provides a flexible mechanism for mobile nodes to communicate their current reachability information to their home agent.
• It is the method by which mobile nodes request forwarding services when visiting a foreign network, inform their home agent of their current care-of address, renew a registration which is due to expire, and / or deregister when they return home.
• Mobile IP defines two different registration procedures, one via a foreign agent that relays the registration to the mobile node’s home agent, one directly with the mobile node’s home agent (co-located care-of address). If a mobile node is registering a foreign agent care-of address, the mobile node must register via that foreign agent.
• If a mobile node is using a co-located care-of address, and receives an Agent Advertisement from a foreign agent on the link on which it is using this care-of address, the mobile node should register via that foreign agent. If a mobile node is otherwise using a co-located care-of address, the mobile node must register directly with its home agent.

Tunneling:

• A reciprocal tunnel is set up by the Home Agent to the care-of address of the current location of the Mobile Node on the foreign network to route packets to the Mobile Node as it roams.
• The start of the tunnel is the home agent, which does the encapsulation. The end of the tunnel depends on what sort of care-of address is being used:
• The foreign agent care-of address is the end of the tunnel. It receives encapsulated messages from the home agent, strips off the outer IP header and then delivers the datagram to the mobile node.
• This is generally done using layer two, because the mobile node and foreign agent are on the same local network, and of course, the mobile node does not have its own IP address on that network.
• The co-located care-of address is the mobile node itself that is the end of the tunnel which strips off the outer header.